Do you ever enjoy being sad? Like it becomes a comfort of some sort or puts you in an interesting mindset (and I don't mean it in a bad way at all). You feel more contemplative.

It's like listening to a sad song, and rejoicing in it because you feel comfort at the way it makes you feel.

I could go even so far to say that on occasion it benefits work. If you're doing creative work, the feeling of being unhappy might make you reflect more on things, and make certain ideas more important.

After so long of feeling sad it becomes normal, as a human you just learn to accept it for survival, you program your brain to live with it.